Method and apparatus for cementing a well conduit

ABSTRACT

A method and apparatus for cementing a well conduit disposed within a well borehole extending into a subterranean earth formation, the conduit being open at the bottom thereof and having drilling mud therein and an annulus formed between the conduit and the well borehole. A solidifiable slurry is pumped down the well conduit thereby displacing the drilling mud out the bottom of the conduit and up the annulus. A displacement fluid is pumped down the well conduit thereby displacing the solidifiable slurry out the opening at the bottom of the conduit during the displacement of at least an initial portion of the solidifiable slurry.

0 Muted Sites tent [151 9 Smith 1 Feb. 1, 1972 [54] METHOD AND APPARATU FUR 3,123,517 3/1964 Davis 166/224 CEMENTING A WELL CONDUIT 3,126,060 3/ 1964 Loiacano 1 66/224 [72] Inventor: Hubert Irvin Smith, Houston, Tex. Primary Examiner stephen L Novosad [73] Assignee: Shell 011 Company, New York, N.Y. Attorney-Louis J. Bovasso and J H. McCarthy [22] Filed: Feb. 11, 1970 [57] ABSTRACT 21 A l. 1 PP No 109543 A method and apparatus for cementing a well conduit disposed within a well borehole extending into a subterranean [52] US. Cl. ..l66/291, 166/ 153, 166/224 earth formation, the conduit being open at the bottom thereof [51] Int. Cl. "E2193 33/13, EZlb 33/16 nd having drilling mud therein and an annulus formed Field of emh between the conduit and the well borehole. A soiidifiable slur- 166/ 175/3 17 ry is pumped down the well conduit thereby displacing the 56 R f C ed drilling mud out the bottom of the conduit and up the annulus. 1 e t A displacement fluid is pumped down the well conduit thereby UNITED STATES PATENTS displacing the soliditiable slurry out the opening at the bottom of the conduit during the displacement of at least an initial Owen 1 portion of [he solidifiable ]un'y 2,217,708 10/1940 Scararnucci. .....l66/29l X 2,319,514 5/1943 Penfield ..l75/317 X 6 Claims, 7 Drawing Figures PATENTEDFEB H912- 31638730 SHEEI 2 OF 2 INVENTOR:

HUBERT I. SMITH BY Mg/E In HIS ATTORNEY METHOD AND APPARATUS FOR CEMENTING A WELL CONDUIT BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to completion operations carried out in walls drilled in the earth; and, more particularly, to an ap paratus and method for cementing a conduit such as casing within such wells.

2. Description of the Prior Art After the drilling of an oil or gas well, casing is normally installed within the well to a predetermined depth to prevent the hole from caving and to confine the locations at which fluid can enter or leave the hole. Such casing also provides a firm base and anchor for equipment, such as blowout preventers, etc., which may be installed later at the well.

The casing is rigidly secured in position by filling the annular space between the casing and borehole wall with cement. In the typical cementing operation, a cement slurry is pumped into the casing down to the casing shoe, thereby displacing any mud within the casing down through the bottom thereof. The cement slurry is then normally displaced out the bottom of the casing and up the annulus by means of a suitable displacement fluid. Spacer plugs, maintaining a substantially fluidtight fit with the interior of the casing, usually preceed and follow the slurry, thus separating it from the mud and displacement fluid.

In the event the slurry in the aforementioned operations flows out of the bottom of the casing at too high a rate, laminar flow thereof may result. The parabolic flow profile that results will allow extensive contact of the mud and cement and the resulting mixture may be so viscous as to be unpumpable. The remainder of the slurry will channel through this jelled mass, becoming contaminated with mud chemicals which will retard or prevent setting of the cement. This undesirable high initial slurry flow rate is primarily caused in prior art cementing operations due to the fact that the hydrostatic head of the heavy cement inside the casing exceeds that of mud in the annulus to a maximum extent when the cement slurry column is at its greatest height. The slurry will fall at the rate at which the friction drop equals this hydrostatic head unbalance. This initial high rate of slurry flow is known as the socalled U-tube effect.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide an improved method and apparatus for cementing a well conduit within a well borehole.

A further object of the present invention is to provide an improved cementing apparatus and method whereby the outflow rate of cement slurry from the conduit being installed is restricted during the outflowing of at least the initial portion of the cement.

These and other objects are preferably carried out by pumping a solidifiable slurry down a well conduit thereby displacing drilling mud present in the conduit out the bottom of the conduit and up the annulus formed between the conduit and the well borehole. A displacement fluid is pumped down the conduit thereby displacing the solidifiable slurry out the bottom of the conduit and up the annulus while restricting the size of the opening at the bottom of the conduit during the displacement of at least an initial portion of the solidifiable slurry.

Such restriction of the opening of the conduit may be carried out by providing aperture-defining means at the bottom of the conduit which is constructed of a fluid-destructable material such as lead or aluminum which enlarges as an abrasive and/or dissolving fluid such as a cement slurry containing sand and/or an aqueous alkali passes through and erodes and/or dissolves it until, at the approximate head equalization point, it becomes a full opening. The erodable aperture-defining means may constitute a bottom hole choke arrangement located in the casing shoe or alternatively in the bottom cementing plug. In one embodiment of the invention, the apertune-defining means comprises an erodable plate with a thinning cross section as the diameter thereof increases due to outflowing of the slurry. The erosion may be due to either or both of a wearing away of dissolving action.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 through 3 are crosssectional views illustrating in a schematic fashion three consecutive stages of a cementing operation being carried out according to the method and with the apparatus of the present invention;

FIG. 4 is an enlarged detailed plan view of a plug incorporating erodable aperture-defining means in accordance with the teachings of the present invention;

FIG. 5 is a cross-sectional view taken along the line 5-5 of FIG. 4 and illustrating the plug of FIG. 4 in cooperative relationship with the bottom of a casing;

FIG. 6 is a view similar to that of FIG. 5 but illustrating the aperture-defining means of a plug in an eroded condition; and

FIG. 7 is a longitudinal view taken in partial cross section illustrating details of a casing shoe incorporating erodable aperture-defining means.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the initial stage of a typical cementing operation is illustrated in schematic fashion. In FIG. 1, a conduit such as surface casing 11 is illustrated as being positioned within well borehole 12. It is, of course, to be understood that surface casing 11 is maintained in such position by means of conventional casing support equipment such as slips 13 disposed between casing head 14 and casing 11 in a well-known manner. After surface casing 11 has been suspended within the well borehole, mud 15 is conventionally circulated down the casing out the bottom thereof, and up into the annulus 16, which is formed between the suspended casing 11 and the wall of well borehole 12. Prior to the introduction of the cement slurry into the well casing, a bottom spacer plug 17 is situated within the casing above the mud 15.

Referring now to FIGS. 4 and 5, a form of bottom spacer plug 17 which may be suitably utilized in carrying out the teachings of the present invention is illustrated. As may be clearly seen with reference to these figures, bottom spacer plug 17 has a circular configuration. This configuration substantially conforms to the inner dimensions of surface casing 11. The plug comprises a main body portion I8 (FIG. 5) provided with a centralized throughbore l9 and an upper plate portion 18a. It should be noted that main body portion 18 is domed downwardly as at 20 with this domed segment comprising the means defining throughbore 19. The configuration of dome 20 is such that body portion 18 has a thinning cross section in the center thereof when moving in the direction away from centralized throughbore 19. The purpose of this configuration will be brought out in greater detail below. The upper plate portion 18a is preferably of a sufficient thickness to preclude shear failure. Secured in any desired manner to the top of main body portion 18 is a diaphragm plate 21 constructed of steel or other similar suitable material, and having a circular configuration approximating that of the main body portion 18. It should be noted that diaphragm plate 21 wholly extends over centralized throughbore 19 so that fluid flow through the throughbore 19 is prevented as long as the plate 21 remains intact. A plurality of scorelines may be scribed in the plate 21 as at 22 (FIG. 4) to provide a scored area approximating the configuration of throughbore 19.

Referring once again to FIG. 1, after the bottom spacer plug 17 has been positioned above mud 15, a slurry of solidifiable fluid, such as a cement slurry 23, is pumped into the casing 11 above bottom spacer plug 17 through an entry conduit 24 which communicates with the interior of the casing 11. Slurry 23 is introduced under pressure into the entry conduit 24 by means of conventional cement-pumping apparatus operatively associated with a suitable source of supply (both of the latter elements not being illustrated for purposes of simplicity). Pumping of the cement slurry 23 continues until the desired amount has been introduced into the casing 11. This slurrypumping operation forces mud contained within the casing 1 l in a downward direction and out an orifice 31 defined at the bottom of the casing by means of a suitable casing shoe or float collar 32, which is illustrated in schematic form.

After the desired amount of slurry 23 has been pumped into easing 1 1, a top plug 33 (FIG. 2) which may be of any suitable conventional design, is introduced into the casing 11 so that it floats upon slurry 23. At this point, a suitable displacement fluid 34, which may be mud or water, for example, is pumped into entry conduit 24 to pump top plug 33 and slurry 23 down the surface casing 1 1.

FIG. 2 illustrates conditions as they exist just before bottom plug 17 arrives at the casing shoe or float collar 32. At this point during the operation, a zero pressure exists at the upper portion of the casing in the vicinity of entry conduit 24. This occurs because the hydrostatic head of the heavy cement slurry inside the casing exceeds that of the mud in the annulus 16. The slurry will fall at the rate at which the friction drop equals this hydrostatic head unbalance. This rate may be in the order of 15 barrels per minute, for example. Fast displacement up to this point is not necessarily undesirable, for it tends to create turbulent flow of the mud in the annulus 16, thus helping to insure that no pockets of jelled mud exist in the region soon to be covered by cement.

As plug 17 abuts against casing shoe 32, as illustrated in FIGS. 2 and 3,. diaphragm plate 21 ruptures along the scorelines 22 with that portion of the plate 21 within the scorelines functioning as a rupture disc which breaks away under the pressure of the cement slurry 23 and displacement fluid 34 when the plug 17 hits the shoe 32. This allows the slurry 23 to begin passing through throughbore l9 and orifice 31 of shoe 32 into the annulus 16. At this instant, with most of the slurry 23 still inside the casing 11, the flow rate of the slurry is high. This high rate of slurry flow leads to the undesirable socalled U-tube effect. If the rate at which the slurry 23 exits from the casing shoe 32 is high enough to cause turbulent flow, no harm will be done since most of the mud will be removed. If the slurry 23 moves in laminar flow, however, the

- parabolic flow profile that results will allow extensive contact of the mud and cement within annulus 16. The resulting mixture may be so viscous as to be virtually unpumpable. The remainder of the slurry 23 will channel through this jelled mass, being contaminated with mud chemicals which will retard or prevent setting of the cement within the annulus 11. This U-tube effect will be ceased by the time the cement columns equalize.

in accordance with the present invention, U-tubing" is prevented due to the fact that throughbore 19, at least in the initial phases of outflow of the cement slurry 23, is of a relativeiy small cross section in comparison with the dimensions of orifice 31 defined by the casing shoe 32. The main body portion 18 of bottom spacer plug 17 in essence constitutes a choke-sized tube to allow a reduced flow rate. Obviously, the dimensions of centralized throughbore 19 must be chosen for the pressure conditions of each individual well in which a spacer plug similar to plug 17 is utilized, taking into consideration the flow properties of the pseudoplastic cement slurry.

As the cement slurry 23 flows outwardly through throughbore l9 and orifice 31 of shoe 32 and up the annulus 16, the rate of flow gradually decreases until the head pressure of the material inside the casing 11 is substantially equal to that of the material in the annulus 16. Since considerable amounts of cement slurry may be utilized in the operation, it is desirable to provide some means whereby introduction of cement slurry 23 into the annulus 16 is speeded up as the cement columns progressively move toward equalization pressures. As the cement column fills within the casing 11, displacement fluid 34 is pressured up within the casing 11 to assist in the outward flow of the slurry 23. However, any pressure drop involved in the outflow of the slurry 23 must be added to the pressure required to lift the cement column in the annulus 16. Such total pressure must be maintained below the casings working pressure. To insure this, at least the central domed portion 20 of bottom spacer plug 17 is constructed of an erodable material, such as lead, which would erode away as the slurry passed through it, thus wiping away the orifice I9 and plate portion 18a in the manner illustrated in FIG. 6. lt should be noted that diaphragm plate 21 has also eroded away under the action of the slurry material, since it is of relatively thin construction. The bottom spacer plug design should be such that throughbore 19 would enlarge as the slurry passed through it until it would be at full opening at the time of the approximate equalization point. it is desirable that the orifice pressure drop should decline uniformly during this period. Since differential pressure is universally proportional to the fourth power of the diameter of the throughbore, the diameter should enlarge slowly at first, and at a faster rate later. This may be accomplished by means of the domed construction 20 of the spacer plug 17 wherein the erodable part of main body portion 18 has a thinning cross section as the diameter of throughbore 19 increases.

Rather than establishing the erodable outflow throughbore defining means 19 in the bottom spacer plug 17, the erodable throughbore defining means 19 may be operatively associated directly with the casing shoe itself in the manner illustrated in FIG. 7. With reference to that figure, it should be noted that the material of easing shoe 320 immediately surrounding outflow throughbore 19a is constructed of an erodable material such as lead, as at 35. It is, of course, to be understood that the slurry 23 passing through throughbore will erode away the inner portion 35 in much the same manner as that previously described with respect to the bottom spacer plug arrangement. A disadvantage to placing the choke or throughbore defining means into the casing shoe itself is the large pressure drop required to circulate mud at normal rates when washing to bottom, conditioning the hole, and mixing cement. For example, in the event a iii-inch throughbore is employed under 3,000-p.s.i. differential operating pressures, the throughbore will pass only 4.7 barrels per minute of l0-p.p.g. mud and 3.9 barrels per minute of l5-p.p.g. mud. These pressure rate combinations are not generally considered suitable. While the plug arrangement illustrated in N68. 4, 5 and 6 has the possible disadvantage of shock-loading, occuring when the free-faliing cement first hits bottom, this effect may not be appreciably greater than when a standard bottom plug is ruptured or when a top plug is bumped with the pumps wide open. The calculated pressure increase from the water-hammer effect of instantaneously and completely stopping a IS-barrel-per-min ute flow of l5-pound-per-gallon slurry in 7-inch 26-pound slant foot casing is about 1,000 p.s.i. This, coupled with a somewhat similar rarefaetion in the annulus, gives rise to not-insignificant pressure differentials across the casing wall. These pressures are, however, within the working pressure of most well casings.

I claim as my invention:

1. A method for cementing a well conduit disposed within a well borehole extending into a subterranean earth formation, said conduit being open at the bottom thereof and having fluid therein and an annulus formed between said conduit and said well borehole, said method comprising the steps of:

pumping a solidifiable slurry down said well conduit thereby displacing said fluid out the bottom of said conduit and up said annulus;

pumping a displacement fluid down said well conduit thereby displacing said solidifiable slurry out the bottom of said conduit and up said annulus;

temporarily restricting the size of the opening at the bottom of said conduit while an initial portion of said solidifiable slurry is being displaced out the bottom of said conduit and up said annulus; and

continuing said restricting of the size of the opening at said conduit until the hydrostatic pressure of the slurry and displacement fluid present in said conduit is substantially equal to that of the fluid and slurry present in said annulus.

2. The method of claim 1 wherein the steps of temporarily restricting the size of the opening includes the steps of:

closing ofi' the opening at the bottom of said conduit prior to pumping said slurry down through said opening and up the annulus; and

pumping a slurry-erodable aperture-defining means adapted to restrict the opening at the bottom of said conduit down the conduit ahead of said slurry.

3. Apparatus for controlling the rate at which a solidifiable slurry is circulated into the annulus formed between a well conduit disposed in a well borehole extending into a subterranean earth formation and said borehole, said apparatus comprising:

a well conduit being cemented within a well having an opening at the bottom thereof;

aperture-defining means associated with said well conduit and adapted to restrict the opening at the bottom thereof; and

slurry-erodable opening means associated with said aperture-defining means for increasing the size of restricted opening under the action of said slurry passing therethrough when said slurry is being circulated down said well conduit out the bottom opening thereof and up said annulus, whereby the size of said opening and the rate of flow of said slurry is restricted temporarily, during the initial portion of said slurry circulation.

4. The apparatus of claim 3 wherein said aperture-defining means comprises a conduit shoe closing off a portion of the bottom of said conduit; and

said slurry-erodable opening means comprises erodable elements integral with said shoe and forming an opening therein.

5. The apparatus of claim 3 wherein said conduit has a conduit shoe closing off a portion of the bottom thereof and said slurry-erodable opening means comprises a plug having an opening therein adapted to rest on the top of said shoe; and

said plug includes a rupturable diaphragm disposed over the opening therein.

6. The apparatus of claim 5 wherein said plug includes an erodable plate having a thinning cross section from its opening outwardly as the diameter thereof increases due to the flowing therethrough of said slurry. 

2. The method of claim 1 wherein the steps of temporarily restricting the size of the opening includes the steps of: closing off the opening at the bottom of said conduit prior to pumping said slurry down through said opening and up the annulus; and pumping a slurry-erodable aperture-defining means adapted to restrict the opening at the bottom of said conduit down the conduit ahead of said slurry.
 3. Apparatus for controlling the rate at which a solidifiable slurry is circulated into the annulus formed between a well conduit disposed in a well borehole extending into a subterranean earth formation and said borehole, said apparatus comprising: a well conduit being cemented within a well having an opening at the bottom thereof; aperture-defining means associated with said well conduit and adapted to restrict the opening at the bottom thereof; and slurry-erodable opening means associated with said aperture-defining means for increasing the size of restricted opening under the action of said slurry passing therethrough when said slurry is being circulated down said well conduit out the bottom opening thereof and up said annulus, whereby the size of said opening and the rate of flow of said slurry is restricted temporarily, during the initial portion of said slurry circulation.
 4. The apparatus of claim 3 wherein said aperture-defining means comprises a conduit shoe closing off a portion of the bottom of said conduit; and said slurry-erodable opening means comprises erodable elements integral with said shoe and forming an opening therein.
 5. The apparatus of claim 3 wherein said conduit has a conduit shoe closing off a portion of the bottom thereof and said slurry-erodable opening means comprises a plug having an opening therein adapted to rest on the top of said shoe; and said plug includes a rupturable diaphragm disposed over the opening therein.
 6. The apparatus of claim 5 wherein said plug includes an erodable plate having a thinning cross section from its opening Outwardly as the diameter thereof increases due to the flowing therethrough of said slurry. 